According to the Sporting News Record Book, the NL record for SLG by a rookie in 100+ games is .621 by George Watkins in 1930. The AL record is .618 by McGwire in 1987. As of last night, Braun's SLG is .643. If he plays every game the rest of the season, he needs to slug .575 the rest of the way to finish at .622 and he would then have the record. In the games since August 14, when his SLG was at .684, he has slugged just .214. But he slugged .716 in June and .673 in July. Even with his currect slump, his August SLG is still .600. So I think that he will come out of it and slug well enough the rest of the way to break the record.

The SABR Baseball List and Record Book lists the best slugging percentages by rookies who qualified for the league lead. Here they are. The number after the year is the player's rank if he was in the top 5.

1. Mark McGwire-.618 (1987,1)
   
2. Wally Berger-.614 (1930)
   
3. Albert Pujols-.610 (2001)
   
4. Ted Williams-.609 (1939,4)
   
5. Hal Trosky-.588 (1934,4)
   
6. Walt Dropo-.583 (1950,2)
   
7. Dale Alexander-.580 (1929,5)
   
8. Johnny Mize-.577 (1936,3)
   
9. Joe DiMaggio-.576 (1936)
   
10. Mike Greenwell-.570 (1987)
    
11. Fred Lynn-.566 (1975,1)
    
12. Mike Piazza-.561 (1993,4)
    
13. Frank Robinson-.558 (1956,4)
    
14. Dick Allen-.557 (1964,3)
    
15. Tony Oliva-.557 (1964,3)
    
16. Rico Carty-.554 (1964,4)
    
17. Jason Bay-.550 (2004)

(Watkins did not qualify in 1930). 1964 must have been the golden year of the rookie slugger. It was also the rookie year of Tony Conigliaro (he slugged .530 although he did not qualify for the league lead). Allen was 3rd in the NL and Carty was 4th. Oliva was 3rd in the AL. But looking at this list and seeing several Hall of Famers and others who had very long and productive careers, we should expect big things from Braun. Alexander only played 5 years but he did win a batting title and had a lifetime average of .331. Dropo played 13 years but his rookie year was the only great one and his lifetime offensive winning percentage was only .450.

If Braun had enough plate appearances right now, he would be leading the league. If he plays every game the rest of the way, he might get 502 PAs and could finish in first. But whatever happens, he will likely crack the above list and that is usually a good sign.